Medial triangle

Each side of the medial triangle is called a midsegment (or midline).

In general, a midsegment of a triangle is a line segment which joins the midpoints of two sides of the triangle.

It is parallel to the third side and has a length equal to half the length of the third side.

The medial triangle can also be viewed as the image of triangle △ABC transformed by a homothety centered at the centroid with ratio -1/2.

Thus, the sides of the medial triangle are half and parallel to the corresponding sides of triangle ABC.

Hence, the medial triangle is inversely similar and shares the same centroid and medians with triangle △ABC.

It also follows from this that the perimeter of the medial triangle equals the semiperimeter of triangle △ABC, and that the area is one quarter of the area of triangle △ABC.

Furthermore, the four triangles that the original triangle is subdivided into by the medial triangle are all mutually congruent by SSS, so their areas are equal and thus the area of each is 1/4 the area of the original triangle.

[1]: p.177 The orthocenter of the medial triangle coincides with the circumcenter of triangle △ABC.

This fact provides a tool for proving collinearity of the circumcenter, centroid and orthocenter.

The nine-point circle circumscribes the medial triangle, and so the nine-point center is the circumcenter of the medial triangle.

The Nagel point of the medial triangle is the incenter of its reference triangle.

[3]: p.233, Lemma 1 A point in the interior of a triangle is the center of an inellipse of the triangle if and only if the point lies in the interior of the medial triangle.

Trilinear coordinates for the vertices of the medial triangle

is formed by three lines parallel to the sides of

Trilinear coordinates for the vertices of the triangle

are given by The name "anticomplementary triangle" corresponds to the fact that its vertices are the anticomplements of the vertices

The vertices of the medial triangle are the complements of